The effects of thermal exposure on microstructure of an experimental third generation single crystal superalloy were investigated. The alloy was exposed at 1100℃ for 100-800 h. The γ′ particles in dendrite core gradually had coarsened and became very irregular by γ′ precipitates dissolving resulting from elements diffusion with increasing exposure time. The γ′ precipitates in interdendritic region varied from cuboidal morphology, fusing together, and rafting with the increase of exposure time. The various γ′ particles behaviors between dendritic core and interdendritic region were attributed to the segregation of refractory elements. After 300h a small amount of needle-shaped TCP phases were found in dendrite core and the mount of TCP phases increased with increasing exposure time. There was no TCP phase in interdendritic region until 800h. TEM and EDX measurements showed that TCP phases observed in these samples were identified as σ phase enriched Re and W elements.